Nb Doping Induced the Formation of Protective Layer to Improve the Stability of Fe-Ni3S2 for Seawater Electrolysis

The seawater electrolysis to produce hydrogen is a significant topic on alleviating the energy crisis. Here, the Fe, Nb-Ni3S2 catalyst is prepared by metal-doping strategy, and it shows high oxygen evolution reaction (OER) activity in alkaline medium, and only needs 1.491 V to deliver a current density of 100 mA cm-2 in simulated seawater. Using Fe, Nb-Ni3S2 as a bifunctional catalyst, the two-electrode electrolyzer only requires a voltage of 1.751 V (without impedance compensation) to drive the current density of 50 mA cm-2, and can run over 150 h stably in the simulated seawater. Importantly, In situ Raman test demonstrates that the outstanding performance of Fe, Nb-Ni3S2 in simulated seawater is ascribed to the in situ formed sulfate protective layer induced by Nb doping, which can effectively inhibit the corrosion of chloride ion, while the protective layer is absent for Fe-Ni3S2. The stable operation of simulated seawater electrolysis under industrial current density further confirms the stability improvement mechanism of forming protective layer. In short, this study provides a new strategy of using Nb dopants inducing the formation of protective layer to enhance the stability of seawater electrolysis.

Insight into the effective electrocatalytic sulfide removal from aqueous solutions using surface oxidized stainless-steel anode and its desulfurization mechanism

The electrochemical oxidation of hydrogen sulfide (H2S) has shown its potential for the real application of H2S emission control in wastewater treatment. In this study, a surface corrosion treatment of stainless steel (SS) was optimized by regulate Ni content in the oxide film on the SS AISI 304 surface for sulfide removal. The X-ray photoelectron spectroscopy and linear sweeping voltammetry results indicated a higher Ni content in the oxide film of surface-oxidized stainless steel (SOSS) attributed to a higher sulfide removal potential. Sulfide removal experiment results showed that SS-150 (with 150 s anodic pretreatment) anodes achieved the highest Ni content of 69% with the best sulfide removal efficiency, i.e., 97% within 48 h, which increased by 20% compared to the untreated SS. This study also demonstrated a strategy for in situ removal of deposited sulfur on the anodes by cathodic treatment at -0.38 V vs. RHE to alleviate the common issue of sulfur passivation. Density functional theory (DFT) calculation revealed that NiOOH was the major active species in SS-150 oxide film for a faster sulfide removal rate. The study developed a SS surface modification process for Ni content regulation that contributed to better sulfide removal efficiency.

Ce Site in Amorphous Iron Oxyhydroxide Nanosheet toward Enhanced Electrochemical Water Oxidation

Iron oxyhydroxide has been considered an auspicious electrocatalyst for the oxygen evolution reaction (OER) in alkaline water electrolysis due to its suitable electronic structure and abundant reserves. However, Fe-based materials seriously suffer from the tradeoff between activity and stability at a high current density above 100 mA cm^-2 . In this work, the Ce atom is introduced into the amorphous iron oxyhydroxide (i.e., CeFeO_x H_y ) nanosheet to simultaneously improve the intrinsic electrocatalytic activity and stability for OER through regulating the redox property of iron oxyhydroxide. In particular, the Ce substitution leads to the distorted octahedral crystal structure of CeFeO_x H_y , along with a regulated coordination site. The CeFeO_x H_y electrode exhibits a low overpotential of 250 mV at 100 mA cm^-2 with a small Tafel slope of 35.1 mVdec^-1 . Moreover, the CeFeO_x H_y electrode can continuously work for 300 h at 100 mA cm^-2 . When applying the CeFeO_x H_y nanosheet electrode as the anode and coupling it with the platinum mesh cathode, the cell voltage for overall water splitting can be lowered to 1.47 V at 10 mA cm^-2 . This work offers a design strategy for highly active, low-cost, and durable material through interfacing high valent metals with earth-abundant oxides/hydroxides.

Theoretical Prediction of a Bi-Doped β-Antimonene Monolayer as a Highly Efficient Photocatalyst for Oxygen Reduction and Overall Water Splitting

The photo-/electrocatalysts with high activities for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) are of significance for the advancement of photo-/electrochemical energy systems such as solar energy to resolve the global energy crisis, reversible water electrolyzers, metal-air batteries, and fuel cells. In the present work, we have systematically investigated the photochemical performance of the 2D β-antimonene (β-Sb) monolayer. From density functional theory investigations, β-Sb with single-atom doping possesses a trifunctional photocatalyst with high energetics and thermal stabilities. In particular, it is predicted that the performance of the HER activity of β-Sb will be superior to most of the 2D materials. Specifically, β-Sb with single atom replacement has even superior that the reference catalysts IrO2(110) and Pt(111) with relatively low overpotential values for ORR and OER mechanisms. The superior catalytic performance of β-Sb has been described by its electronic structures, charge transfer mechanism, and suitable valence and conduction band edge positions versus normal hydrogen electrode. Meanwhile, the low overpotential of multifunctional photocatalysts of the Bi@β-Sb monolayer makes them show a remarkable performance in overall water splitting (0.06 V for HER, 0.25 V for OER, and 0.31 V for ORR). In general, the Bi@β-Sb monolayer may be an excellent trifunctional catalyst that exhibits high activity toward all electrode reactions of hydrogen and oxygen.

The Critical Role of Additive Sulfate for Stable Alkaline Seawater Oxidation on Nickel-Based Electrodes

Seawater electrolysis to produce hydrogen is a critical technology in marine energy projects; however, the severe anode corrosion caused by the highly concentrated chloride is a key issue should be addressed. In this work, we discover that the addition of sulfate in electrolyte can effectively retard the corrosion of chloride ions to the anode. We take nickel foam as the example and observe that the addition of sulfate can greatly improve the corrosion resistance, resulting in prolonged operating stability. Theoretical simulations and in situ experiments both demonstrate that sulfate anions can be preferentially adsorbed on anode surface to form a negative charge layer, which repulses the chloride ions away from the anode by electrostatic repulsion. The repulsive effect of the adsorbed sulfate is also applicable in highly-active catalyst (nickel iron layered double hydroxide) on nickel foam, which shows ca. 5 times stability of that in traditional electrolyte.

Benchmarking Three Ruthenium Phosphide Phases for Electrocatalysis of the Hydrogen Evolution Reaction: Experimental and Theoretical Insights

The outstanding electrocatalytic activity of ruthenium (Ru) phosphides toward the hydrogen evolution reaction (HER) has received wide attention. However, the effect of the Ru phosphide phase on the HER performance remains unclear. Herein, a two-step method was developed to synthesize nanoparticles of three types of Ru phosphides, namely, Ru₂ P, RuP, and RuP₂ , with similar morphology, dimensions, loading density, and electrochemical surface area on graphene nanosheets by simply controlling the dosage of phytic acid as P source. Electrochemical tests revealed that Ru₂ P/graphene shows the highest intrinsic HER activity, followed by RuP/graphene and RuP₂ /graphene. Ru₂ P/graphene affords a current density of 10 mA cm^-2 at an overpotential of 18 mV in acid media. Theoretical calculations further showed that P-deficient Ru₂ P has a lower free energy of hydrogen adsorption on the surface than other two, P-rich Ru phosphides (RuP, RuP₂ ), which confirms the excellent intrinsic HER activity of Ru₂ P and is consistent with experiment results. The work reveals for the first time a clear trend of HER activity among three Ru phosphide phases.

Free-Sustaining Three-Dimensional S235 Steel-Based Porous Electrocatalyst for Highly Efficient and Durable Oxygen Evolution

A novel oxygen evolution reaction (OER) catalyst (3 D S235-P steel) based on a steel S235 substrate was successfully prepared by facile one-step surface modification. The standard carbon-manganese steel was phosphorized superficially, which led to the formation of a unique 3 D interconnected nanoporous surface with a high specific area that facilitated the electrocatalytically initiated oxygen evolution reaction. The prepared 3 D S235-P steel exhibited enhanced electrocatalytic OER activities in the alkaline regime, as confirmed by a low overpotential (326 mV at a 10 mA cm^-2 ) and a small Tafel slope of 68.7 mV dec^-1 . Moreover, the catalyst was found to be stable under long-term usage conditions, functioning as an oxygen-evolving electrode at pH 13, as evidenced by the sufficient charge-to-oxygen conversion rate (faradaic efficiency: 82.11 and 88.34 % at 10 and 5 mA cm^-2 , respectively). In addition, it turned out that the chosen surface modification delivered steel S235 as an OER electrocatalyst that was stable under neutral pH conditions. Our investigation revealed that the high catalytic activities likely stemmed from the generated Fe/(Mn) hydroxide/oxohydroxides generated during the OER process. Phosphorization treatment therefore not only is an efficient way to optimize the electrocatalytic performance of standard carbon-manganese steel but also enables for the development of low-costing and abundant steels in the field of energy conversion.

A bimetallic oxide Fe1.89Mo4.11O7 electrocatalyst with highly efficient hydrogen evolution reaction activity in alkaline and acidic media

Transition-metal Mo-based materials have been considered to be among the most effective hydrogen evolution reaction (HER) electrocatalysts. Regulating the electronic structure of Mo atoms with guest metal atoms is considered as one of the important strategies to improve their HER activity. However, introduction of guest metal elements in the vicinity of Mo sites with atomic-level hybridization is difficult to realize, resulting in the failure of the modified electronic structure of Mo sites. Herein, an Fe1.89Mo4.11O7/MoO2 material is prepared through the thermal treatment of a ferrimolybdate precursor. It exhibits a Tafel slope of 79 mV dec-1 and an exchange current density of 0.069 mA cm-2 in 1 M KOH medium, as well as a Tafel slope of 47 mV dec-1 and an exchange current density of 0.072 mA cm-2 in 0.5 M H2SO4 medium. Compared to original Mo-based oxides, Fe1.89Mo4.11O7 with the regulated Mo electronic structure shows a more suitable Mo-H bond strength for the fast kinetics of the HER process. Density functional theory (DFT) calculations also indicate that the Mo-H bond strength in Fe1.89Mo4.11O7 is similar to the Pt-H bond strength, resulting in the high kinetic activity of Mo-based HER electrocatalysts in alkaline and acidic media.

Advanced Architectures and Relatives of Air Electrodes in Zn-Air Batteries

Zn-air batteries are becoming the promising power sources for portable and wearable electronic devices and hybrid/electric vehicles because of their high specific energy density and the low cost for next-generation green and sustainable energy technologies. An air electrode integrated with an oxygen electrocatalyst is the most important component and inevitably determines the performance and cost of a Zn-air battery. This article presents exciting advances and challenges related to air electrodes and their relatives. After a brief introduction of the Zn-air battery, the architectures and oxygen electrocatalysts of air electrodes and relevant electrolytes are highlighted in primary and rechargeable types with different configurations, respectively. Moreover, the individual components and major issues of flexible Zn-air batteries are also highlighted, along with the strategies to enhance the battery performance. Finally, a perspective for design, preparation, and assembly of air electrodes is proposed for the future innovations of Zn-air batteries with high performance.

Dual-Native Vacancy Activated Basal Plane and Conductivity of MoSe2 with High-Efficiency Hydrogen Evolution Reaction

Although transition metal dichalcogenide MoSe₂ is recognized as one of the low-cost and efficient electrocatalysts for the hydrogen evolution reaction (HER), its thermodynamically stable basal plane and semiconducting property still hamper the electrocatalytic activity. Here, it is demonstrated that the basal plane and edges of 2H-MoSe₂ toward HER can be activated by introducing dual-native vacancy. The first-principle calculations indicate that both the Se and Mo vacancies together activate the electrocatalytic sites in the basal plane and edges of MoSe₂ with the optimal hydrogen adsorption free energy (ΔG_H* ) of 0 eV. Experimentally, 2D MoSe₂ nanosheet arrays with a large amount of dual-native vacancies are fabricated as a catalytic working electrode, which possesses an overpotential of 126 mV at a current density of 100 mV cm^-2 , a Tafel slope of 38 mV dec^-1 , and an excellent long-term durability. The findings pave a rational pathway to trigger the activity of inert MoSe₂ toward HER and also can be extended to other layered dichalcogenide.

A Highly Efficient Oxygen Evolution Catalyst Consisting of Interconnected Nickel-Iron-Layered Double Hydroxide and Carbon Nanodomains

In this work, a one-pot solution method for direct synthesis of interconnected ultrafine amorphous NiFe-layered double hydroxide (NiFe-LDH) (<5 nm) and nanocarbon using the molecular precursor of metal and carbon sources is presented for the first time. During the solvothermal synthesis of NiFe-LDH, the organic ligand decomposes and transforms to amorphous carbon with graphitic nanodomains by catalytic effect of Fe. The confined growth of both NiFe-LDH and carbon in one single sheet results in fully integrated amorphous NiFe-LDH/C nanohybrid, allowing the harness of the high intrinsic activity of NiFe-LDH due to (i) amorphous and distorted LDH structure, (ii) enhanced active surface area, and (iii) strong coupling between the active phase and carbon. As such, the resultant NiFe-LDH/C exhibits superior activity and stability. Different from postdeposition or electrostatic self-assembly process for the formation of LDH/C composite, this method offers one new opportunity to fabricate high-performance oxygen evolution reaction and possibly other catalysts.

Highly efficient electrocatalytic oxidation of urea on a Mn-incorporated Ni(OH)2/carbon fiber cloth for energy-saving rechargeable Zn–air batteries

The introduction of Mn can effectively regulate the electronic structure of Ni hydroxides on CFC, exhibiting superior electrocatalytic oxidation activity toward urea and potential applications in energy-saving rechargeable Zn–air batteries.